The optic tectum is a structure common to the visual system of all vertebrates. It is the primary center for visual perception, visually guided behavior, and possibly for visual learning in lower vertebrates. Because of its highly laminar organization, the optic tectum is particularly well-suited to electrophysiological studies, although its detailed functional circuitry is unknown. The principal objectives of this study are: (1) to elucidate, by combined anatomical, physiological and behavioral studies, the detailed functional circuitry of the optic tectum of a representative vertebrate, Bufo marinus; (2) to determine the neuronal mechanisms by which some visual sensations are abstracted from the external world and transformed so as to provide the basis for the animal's visually guided behavior. The anatomical study will include a light microscopic examination of Golgi material, and an electronmicroscopic examination of tectal synaptology. The physiological study will consist of a) a quantitative single unit analysis of receptive field properties of different tectal neurons, and b) a newly developed current source-density analysis. The role of the tectum in visuo-motor behavior will be studied in two ways: by recording from tectal neurons in unrestrained animals during conditioned movements, and by observing the effects produced by patterned focal stimulation.